1. Field of the Invention
This invention relates, generally, to munitions. More particularly, it relates to a command to arm fuze that requires no electrical or electronic components.
2. Description of the Prior Art
Modern exploding munitions or rounds are required to carry insensitive explosives that have been specially formulated to prevent explosion resulting from exposure of the munitions or rounds to fire or mechanical abuse during transportation, storage, carrying in the field or any other environment they may encounter up until the moment they are fired from a weapon. Since a round must be “sensitive” if it is to explode upon impact with a target, a safe and arm device (SAD) is required to make the round “sensitive” once it has been fired from the weapon. The SAD does this by controlling the alignment of one or two additional explosive components with the main insensitive explosive which forms an explosive “train,” usually located along the centerline of the round.
The first component of an explosive train is a highly sensitive detonator; the second component is a less sensitive lead explosive, and it is not always required. The third component is an even less sensitive main charge. If all of the explosive components are in axial alignment with one another when the projectile hits a target, a firing pin detonates the detonator and the explosion of the detonator causes explosion of the lead charge and the explosion of the lead charge causes explosion of the main charge. The explosive train is interrupted and the round will not explode if the detonator or lead explosive is not in alignment with the main explosive.
It is conventional to align the lead explosive and the main charge on the centerline of the projectile and to position the detonator off center until after the round is fired. Movement of the detonator by the SAD from the off-center, or safe, position to the centerline is called “arming.” The round and its fuze are armed when the detonator is in alignment with the other explosive components.
A SAD and fuze are usually designed to arm at a specific distance (range) from the weapon that fires the round. The arming range must be sufficiently far from the weapon to ensure the safety of the operator should the round hit a target and explode at the exact instant the fuze arms. Due to inherent variations from fuze to fuze, the range at which they arm can vary greatly. After testing multiple rounds, a “no-arm” range and “all-arm” range can be determined for a given type of fuze. Based on the test data, the “no-arm” range is defined statistically as the range from the weapon at which no fuze will ever be armed. The “all-arm” range is defined as that at which all fuzes will be armed. The spread between the no-arm and all-arm ranges can vary greatly. For example, a M549 fuze may vary from sixty feet (60 ft) (no-arm) to two hundred feet (200 ft) (all-arm).
A fuze that has little or no spread between the no-arm and all-arm ranges is defined in the industry as a “command to arm” fuze. There is a great need for a command to arm fuze due to the close engagement distances of urban warfare. Many times an intended target may be beyond the no-arm range, but well within the all-arm range of a fuze. In this case, the gunner cannot rely on the effectiveness of fired ammunition because some of the rounds will not have armed when they hit the intended target. If the all-arm range can be brought closer to the no-arm range, the weapon will be more reliable and useful over its operating distances.
Conventional mechanical fuzes such as the M550, M549 and M549A use a “spinning” rotor as the primary component of the SAD. The rotor spins about a pivot shaft that is offset from the centerline of the round. Centrifugal forces move the rotor radially away from the centerline as the round spins during flight. The CG of the rotor is typically offset from the centerline of the round when the fuze is unarmed in a location that will maximize the centrifugal force on the rotor.
The detonator is mounted within the rotor. The rotor and its pivot location are designed such that the detonator is spaced apart from the centerline when the fuze is unarmed. As the rotor spins about the pivot, the detonator moves to the centerline of the round so that it is aligned with the lead and main explosives. The rotor hits a mechanical stop when it is so aligned. The rotor is locked in the unarmed position by at least two independent safety devices that prevent it from rotating until the round has exited the weapon. A minimum of two safety devices are required by military specifications governing fuzes.
If not restrained, the rotor moves from the unarmed to the armed position in a small fraction of a second on the order of one-thousandth of a second (0.001 s). This unrestrained arming time is due to the centrifugal force resulting from the mass of the rotor and detonator. Given the velocity of a 40 mm high velocity grenade when it exits the weapon, the goal of an SAF is to arm the round in one-tenth of a second (0.1 s) to ensure an arming distance of approximately eighty feet (80 ft). Therefore, unless the rotor is somehow slowed down, the SAD will arm much too quickly.
The speed of the rotor is slowed down by mechanisms that absorb the kinetic energy of the rotor. A classic version of this mechanical absorber is known as a verge and pinion/starwheel. The starwheel is a small rotating disk that is coupled to the rotor via a pinion mounted upon the shaft of the disk. Gear teeth on the rotor spin the starwheel as the rotor moves from the unarmed to the armed position. The spinning starwheel repeatedly strikes a cam, called a verge, causing it to oscillate about a pivot shaft. The starwheel/verge system converts potential energy stored by the rotor to kinetic energy in discrete increments, acting as a brake to slow down the spinning (pivoting) of the rotor. Friction between the various mechanical components of the SAD also absorbs much of the energy of the rotor. Sources of friction include the pivot shafts about which the rotor, starwheel and verge rotate. These shafts are usually positioned radially outwardly from the centerline of the round.
Due to centrifugal forces on the respective CG's of the rotor, starwheel and verge, the loads on these shafts can be as much as fifteen hundred (1,500) times the force of gravity. Unfortunately, friction is not easy to characterize due to its variability in different environments; the coefficient of friction between two materials can vary by as much as 100% over a small temperature range. Moreover, the tolerances of the respective components can dictate how tightly they rub together. Accordingly, small variations in tolerances can result in large variations in the amount of friction. This friction problem cannot be easily addressed by adding lubrication to the system. In fact, adding conventional lubrications such as oil can actually cause the SAD to bind and stop functioning. The only practical means of lubricating the SAD is by the use of small, precise amounts of dry Teflon® powder; however, the powder application method must be tightly controlled or the treated SAD's may bind and not function.
Electronic SAD's have been developed, but they are not yet used in mass quantity production. An electronic timer could be used to initiate the arming very precisely and assure a Command to Arm fuze; however, some actuation system is still required to actually move the detonator from the unarmed to the armed position. Battery shelf life has also been a concern that has yet to be adequately addressed. The largest impediment for electronic fuze acceptance remains the cost of production in large quantities.
However, in view of the prior art taken as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the identified needs could be fulfilled.